Deposition of uniformly-thick films of insulating, semiconducting, and conducting materials is of paramount importance to the microelectronics industry. As the lateral feature size of circuit elements continues to shrink (in order to achieve improved circuit performance), the uniformity tolerances on film thicknesses also scales down proportionally. Conventional film deposition methods such as physical vapor deposition, chemical vapor deposition, and atomic layer deposition can achieve uniform thicknesses (at the precision of single atomic layers) over extremely large areas, however such systems are costly both to purchase and to maintain. Also, there are other lower-performance types of applications for microelectronics where it would be desirable to deposit uniform layers of materials without requiring highly specialized (and expensive) deposition systems.
Chemically-synthesized nanoparticles provide a low-cost alternative route to the production of materials that are highly-uniform in size and composition. High-temperature solution-phase synthesis is one method by which highly-uniform materials can be produced. Methods exist for production of a variety of metals, insulators, and semiconductors. Briefly stated, these methods produce solutions of inorganic nanoparticles with mean diameters tunable through the range of 1 nm to 20 nm and with mean diameter standard deviations on the order of 5%. These nanoparticles are individually coated with organic surfactants that can be tailored to be in the range of 1-4 nm long. The surfactant prevents the nanoparticles from aggregating in solution. A schematic of a chemically-synthesized nanoparticle is shown in FIG. 1(a).
Uniformly-sized nanoparticles can be made to organize themselves into a crystal when deposited from solution onto a substrate. Because of their uniform size, spherically-shaped nanoparticles will pack into hexagonal-close-packed (HCP) arrangements as shown schematically in FIG. 1(b). This process is often referred to as self-assembly. Nanoparticles of other shapes will pack into different crystal arrangements. For example, cubic shaped nanoparticles will pack into a cubic lattice. One advantage of this type of self assembly is that, because of the uniform diameter of the nanoparticles, the resulting film has a very uniform thickness. In other words, films composed of a single layer of nanoparticles will be uniformly one nanoparticle-diameter thick.
Additionally, there is increasing interest in utilizing nanoparticles composed of different materials as catalysts for growth of one dimensional (1-D) materials. This technique involves applying the (typically) metal catalysts to a surface, and then growing the 1-dimensional material using a technique such as chemical vapor deposition. The size of the catalyst will heavily influence the diameter of the resulting 1-D structure. In nearly all cases of growth of this type, the substrate (and catalyst) must be heated to high temperatures (over 400° C. and can be up to 1000° C.). However, at these high temperatures, nanoparticle-type catalysts distributed over a surface of a substrate will often aggregate, resulting in larger-sized catalysts with a broader size distribution (determined by metal diffusion during aggregation), and ultimately larger-diameter 1-D materials with a broader size distribution.
In spite of the intrinsic propensity of nanoparticles to self-organize and the potential advantages of nanoparticle films, there do not exist methods for uniformly depositing nanoparticle films over large areas, similar to conventional film deposition methods such as physical vapor deposition, sputtering, or chemical vapor deposition. Four methods for nanoparticle film deposition have been used to date:
1. Deposition from solution followed by solvent evaporation: In this method a solvent containing dissolved nanoparticles is deposited onto a substrate and the solvent is removed through controlled evaporation. As the solvent evaporates the nanoparticles organize themselves into crystalline layers. This method produces nicely-organized films, but the film thickness is uncontrolled. Layers of varying thickness forms as the solvent evaporates.
2. Nanoparticle film deposition by substrate immersion: In this method the substrate is immersed into a nanoparticle-containing solution and allowed to sit. Over time, nanoparticles diffuse in solution and find their way to the substrate. This method produces films of uniform thicknesses, however nanoparticle layers are not close-packed and often contain voids (regions devoid of nanoparticles). In addition, this method deposits nanoparticle layers everywhere on a surface.
3. Langmuir-Blodgett technique: In this method a nanoparticle film is formed on a liquid surface. By compressing the film on the liquid, the nanoparticles can be made to self organize. Films are transferred to a solid substrate by dip coating onto the liquid surface. This method produces ordered nanoparticle layers, however it is difficult to control film thickness. Often films are composed either of multilayers, or else contain voids. Also, cracks in the film can occur due to the stress of film transfer from liquid to solid substrate.
4. Nanoparticle film deposition by spin-casting: In this method nanoparticle-containing solutions are spin-coated onto a solid substrate. After solvent evaporation, a nanoparticle film remains. Nanoparticle films produced by this method are not well-organized, due to the non-equilibrium nature of the spin-casting process.
All of the above four methods describe depositing nanoparticle films over an entire surface of a substrate.